This invention is related to the field of infrared (IR) optical systems. More particularly, this invention relates to a relatively compact imaging lens arrangement for an infrared optical system featuring two lens elements, a low f-number and diffraction-limited performance. The lens system is particularly suitable for incorporation into an infrared or thermal weapon sight.
Cameras, weapon sights, and other types of optical systems typically incorporate an imaging lens arrangement having a group of lenses which collect light from a scene and direct such light onto a detector placed at the focal plane of the optical system. A variety of lens designs are known in the art. Representative examples are disclosed in the following U.S. Patents: Chen et al., U.S. Pat. No. 7,042,656; Canzek, U.S. Pat. No. 4,738,496; Grey, U.S. Pat. No. 3,784,287; Cooper, U.S. Pat. No. 4,999,005; Ohno, U.S. Pat. No. 5,067,803 and U.S. Pat. No. 5,835,281; Ruben, U.S. Pat. No. 3,944,337; Chipper, U.S. Pat. No. 6,292,293; Betensky, U.S. Pat. No. 4,109,995 and DeJager U.S. Pat. No. 3,912,379. Other U.S. Patents disclosing various lens designs include Baumann, U.S. Pat. No. 5,251,063; Jamieson, U.S. Pat. No. 5,446,581; Watanabe, U.S. Pat. No. 6,507,432; Boutellier, U.S. Pat. No. 4,537,464 and Fjelsted, U.S. Pat. No. 4,380,363.
Many of the prior art lens designs are not particularly suitable for use with thermal or IR sights for small arms, and particularly arms such as rifles carried by a soldier, because they incorporate a relatively large number of lens elements in the design and therefore require too much space, and/or are too heavy. Reducing the number of elements to just a few, such as two or three elements, as disclosed in the Chen et al. or Watanabe references above, allows the designs to be more compact and lighter in weight. The art has also taught that aspheric lens elements can be used in imaging lens arrangements. See, e.g., the Chen et al. reference. However, while the introduction of aspheric lenses may allow more compact lens arrangements to be arrived at, they may introduce unwanted aberrations and make such designs more difficult to achieve diffraction-limited performance.
This invention is ideally suited to the use of uncooled detectors that do not require a rear external cold stop as found in earlier detector assemblies. In the case of this invention, the aperture stop may have any position within the lens assembly.
This invention meets a need in the art for an improved compact objective lens arrangement suitable for imaging in the infrared portion of the spectrum. While there are many possible applications for the lens of this disclosure and the invention in its broader aspects is not limited to any particular application, the lens is especially suitable for use in IR or thermal weapon sights for small arms.